Windows: Add off-screen rendering support (issue #518).

- Popup menus, drag&drop and GPU acceleration are not currently supported.
- Testing is enabled in cefclient by passing the "off-screen-rendering-enabled" command-line flag.

git-svn-id: https://chromiumembedded.googlecode.com/svn/trunk@919 5089003a-bbd8-11dd-ad1f-f1f9622dbc98

include/cef_display_handler.h

@@ -78,6 +78,8 @@ class CefDisplayHandler : public virtual CefBase {
   // text that will be displayed in the tooltip. To handle the display of the
   // tooltip yourself return true. Otherwise, you can optionally modify |text|
   // and then return false to allow the browser to display the tooltip.
+  // When window rendering is disabled the application is responsible for
+  // drawing tooltips and the return value is ignored.
   ///
   /*--cef(optional_param=text)--*/
   virtual bool OnTooltip(CefRefPtr<CefBrowser> browser,